In wiki markup, you can question an uncited claim by inserting a simple {{Citation needed}} tag, or a more comprehensive {{Citation needed|reason=Your explanation here|date=May 2020}}. Alternatively, {{fact}} and {{cn}} will produce the same result. These all display as:

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When to use this tag

A "citation needed" tag is a request for another editor to verify a statement: a form of communication between members of a collaborative editing community. It is never, in itself, an "improvement" of an article. Though readers may be alerted by a "citation needed" that a particular statement is not supported, many readers don't fully understand the community's processes. Not all tags get addressed in a timely manner, staying in place for months or years, forming an ever growing Wikipedia backlog—this itself can be a problem. Best practice recommends the following:

Tag thoughtfully. Avoid "hit-and-run" or pointed tagging. Try to be courteous and consider the hypothetical fellow-editor who will, we hope, notice your tag and try to find the citation you have requested. When adding a tag, ask yourself: Is it clear just what information you want cited? Is the information probably factual? (If it is not, then it needs deletion or correction rather than citation!) Is the knowledge so self-evident that it really does not need to be cited at all? (Some things do not.)

Some tags are inserted by people well placed to find a suitable citation themselves. If this is the case, consider adding these articles to your watchlist or a worklist so that you can revisit the article when you have the opportunity to fix any verifiability problems yourself.

When not to use this tag

Before adding a tag, at least consider the following alternatives, one of which may prove much more constructive:

If you are sure the statement you want to tag is not factual, even if it does not come under either of the preceding headings, it may be more appropriate to simply remove the text (delete it!). Be sure to add a suitable edit summary such as "Very doubtful – please add a citation if you return the content". If the original statement was accurate after all, this gives someone the chance to put it back, hopefully with a proper citation this time.

If a statement sounds plausible, and is consistent with other statements in the article, but you doubt that it is totally accurate, then consider making a reasonable effort to find a reference yourself. In the process, you may end up confirming that the statement needs to be edited or deleted to better reflect the best knowledge about the topic.

If an article, or a section within an article, is under-referenced, then consider adding an {{Unreferenced}}, {{Refimprove}}, or {{Unreferenced section}} tag to the article or section concerned – these tags allow you to indicate more systemic problems to the page.

A reference at the end of a paragraph typically refers to the whole paragraph, and similarly a reference at the end of a sentence may almost always be taken as referring to the whole sentence. If a particular part of a sentence or paragraph seems to require a separate citation, or looks as if it may have been inserted into the text at a sentence or paragraph level, try to check the original reference rather than adding tags to text that may already be well referenced. The extra parameters available in the {{Citation needed span}} template may allow you to indicate which section you want to refer to.

If someone tagged your contributions with a "Citation needed" tag or tags, and you disagree, discuss the matter on the article's talk page. The most constructive thing to do in most cases is probably to supply the reference(s) requested, even if you feel the tags are "overdone" or unnecessary.

Frequently the authors of statements do not return to Wikipedia to support the statement with citations, so other Wikipedia editors have to do work checking those statements. With 391,975 statements that need WP:Verification, sometimes it's hard to choose which article to work on. The tool Citation Hunt makes that easier by suggesting random articles, which you can sort by topical category membership.